The present invention relates to a dispersion type electroluminescent cell which is caused to luminesce by applying an electric field to phosphorescent powder, and also to a method of producing the same.
It has been well known that, when an electric field is applied to phosphorescent powder such as ZnS with manganese diffused therein, the phosphorescent powder luminesces. Electroluminescent cells exploiting this phenomenon or electroluminescence (EL) have been developed as display devices. However, prior-art electroluminescent cells have had various problems, and few have been put into practical use.
FIG. 1 is a sectional view showing the fundamental structure of a typical electroluminescent cell. Numeral 1 designates a transparent electrode which is formed on one surface of a transparent insulating substrate 2 such as a glass substrate or a plastic film substrate. The transparent electrode 1 may be made of a thin film of In.sub.2 O.sub.3, SnO.sub.2 or the like whose sheet resistance is not higher than several k.OMEGA. per cm.sup.2, a thin film of a metal such as gold or palladium, an aluminum foil which is formed into a mesh having apertures, or the like. Numeral 3 indicates a counter electrode, which is constructed of a metal powder of silver or the like dispersed in a binder of an organic polymer or an inorganic material, or a metal sheet of aluminum, copper or the like adhered to an insulating layer 5. An ordinary electroluminescent cell has the following structure. Between the transparent electrode 1 and the opposing counter electrode 3, opposing to there are sandwiched a luminescent layer in which a phosphorescent powder such as ZnS doped with an activator such as copper and manganese and a coactivator such as chlorine is dispersed in an organic polymer binder, and an insulating layer 5 in which a high-permittivity powder such as TiO.sub.2 or BaTiO.sub.3 is dispersed in an organic polymer binder. Further, the entire lamination is covered with a moisture-proof protective film 6 made of polytrifluorochloroethylene, an epoxy resin or the like. As the phosphorescent powder, some cells utilize a rare-earth element, a monovalent metal, a transition metal, etc. When an A.C. voltage is applied across both the electrodes 1 and 3 in the cell of FIG. 1, an electric field corresponding to the magnitude and frequency of the A.C. voltage acts on the luminescent layer 4 to cause it to luminesce. In order to make the luminous intensity high, the following measures can be taken:
(1) The applied voltage can be raised.
(2) The luminescent layer 4 and the thickness of the insulating layer 5 can be reduced.
(3) An organic polymner binder having high permittivity can be used for the luminescent layer 4 as well as the insulating layer 5.
(4) The A.C. frequency can be raised.
However, in raising the voltage or to reduce the thickness of the luminescent layer 4 and the insulating layer 5, dielectric breakdown between the electrodes 1 and 3 may occur. In order to raise the A.C. frequency, a power source needs to be prepared separately, and this is disadvantgeous. Further, when the frequency is varied, the luminescent wavelength becomes different. Accordingly, in order to enhance the luminous intensity without degrading various characteristics of the electroluminescent cell, an organic polymer binder of high permittivity may be used for the luminescent layer 4 as well as the insulating layer 5. Cyanoethylated cellulose or an epoxy resin have heretofore been employed as the organic polymer binder, but such materials have the following disadvantages. Although the cyanoethylated cellulose exhibits a high permittivity, it is weak in film adhesion, and further, it has an inferior heat-proof property and moisture-proof property. Although the epoxy resin is somewhat excellent in its heat-proof property and its moisture-proof property, it exhibits a low permittivity.
Moreover, the phosphorescent powder typically used in the electroluminescent cell has the weak point that, when supplied with a voltage in a moist state, it is decomposed and losses its luminescing function within a very short time. Therefore, even when covered with the moisture-proof protective film 6, the prior-art electroluminescent cell is not totally immune against moisture, and may have a short lifetime and not be highly reliable.